Diaphragm pumps are used extensively, for example, in the field of health care and medicine to transfer gases or fluids of various types. In some cases, the applications or systems require that the gas(es) volume flow rate (Q) in such pumps be relatively ‘low’ (e.g., in the order of about 50 mL/min, at a relative vacuum of about 100 mBar. In many cases, medical (and other miniature) pumps are designed to take samples of, for example, gases for measurement, and such medical pumps typically use small diaphragm pumps in order to produce effective and stable operating conditions. An eccentric drive mechanism is often used to actuate diaphragm pumps.
FIG. 1A shows a principle of a diaphragm pump 100 with eccentric drive mechanism 102. An eccentric shaft 2 inside a bearing 5 (e.g., ball bearing), which is actuated by a drive shaft 4 of an electric motor, displaces a connecting (push) rod (pump shaft 3) in an upward and downward movement (or ‘back-and-forth’ movement). In turn, pump shaft 3 moves a diaphragm 1 inside a pumping chamber 104. By means of the eccentric shaft 2, pump shaft 3 is set into swinging (6) and tilting motion. Eccentric drive mechanism 102 enables the conversion of the moment of rotation of drive shaft 4 into relatively high forces on the upward and downward motion of pump shaft 3. As such, diaphragm pumps with eccentric drives are well suited for producing relatively high pressures within the limits of the load-carrying capacity of the diaphragm material. Another significant advantage of the eccentric drive is that pumps can be operated in every mounting orientation, which facilitates flexibility in the design of pumping systems.
FIG. 1B schematically illustrates a typical diaphragm pump 106 that implements the ‘pumping’ principle described in connection with FIG. 1A. Typically, a motor's cantilever drive shaft 110, which actuates a pump shaft 120, is mechanically supported by and between two bearings (130,140)—one bearing on each side of a rotor 152 of electric motor 150—in order to mechanically support and stabilize rotor 152 by supporting/stabilizing its drive shaft 110. In this configuration, bearing 140 is structurally interposed between rotor 152 and the diaphragm pump's eccentric drive mechanism 160, while bearing 130 is positioned away from eccentric drive mechanism 160, on the other side of rotor 152. (That is, both bearings 130 and 140 are structurally positioned on the same side of rotor's drive shaft 110 with respect, or relative, to eccentric drive mechanism 160.)
Drive shaft 110 is a cantilever shaft, meaning that it has a free end 170. (Free end 170 is not supported by a bearing; i.e., end 170 is ‘bearing-free’.) This means that the motor's drive shaft portion (L0) extending from bearing 140 to eccentric drive mechanism 160 has some degree of freedom (“DOF”). Having a DOF at the ‘pump side’ of the drive shaft is problematic because when drive shaft 110 rotates, it moves pump shaft 120 in a swinging movement (e.g., swinging movement 6, FIG. 1A) that, in turn, applies a counter force on drive shaft 110 at that point. In other words, the swinging movement of pump shaft 120 generates a moment on shaft portion L0 that acts or tends to bend portion L0 of drive shaft 110 and, thus, to transfer a moment to bearing 140. The moment, whose magnitude is a function of the mechanical force 180 that pump shaft 120 applies to shaft end 170 and the shaft length (L0), applies a radial mechanical force on bearing 140. (The mechanical radial force is radially applied from the bearing's center outward.) Consequently, bearing 140, which has to counteract the rotating radial force, wears out relatively fast (e.g., in less than the avowed 20,000 hours in the case of ‘micro-pump’ systems), thus reducing the pump's overall operation time. Attempts to overcome the bearing wearing problem included making the bearing robust or adding bearings to the diaphragm pump, in a way that made the diaphragm pump, as a whole, more robust. These solutions suffer from at least two drawbacks: (1) the capacity of the robust diaphragm pump cannot be reduced to useful low levels, and (2) such diaphragm pumps consume additional electrical power due to the additional weight and friction of the robust bearing(s).
FIG. 1C illustrates distribution of forces acting on motor's bearings during operation of a diaphragm pump. Fo is a force applied by the diaphragm's push rod (rod 3 in FIG. 1A; rod 120 in FIG. 1B) on free end 170 of cantilever shaft 110. R1 and R2 are counteracting forces acting at the bearing nearest to the diaphragm (at location 103) and at the remote bearing (at location 105). Since end 170 of cantilever shaft 110 is not supported by a bearing, it is free to move laterally and exert detrimental (e.g., ‘grinding’) radial forces on the two bearings, as described above. The detrimental radial forces acting on the two bearings depend, among other things, on the values of Fo, R1, R2, Lo and L, as described below. Equations (1) and (2) apply at equilibrium (static state) of the pump:F0*L0=L*R2  (1)R1=F0+R2  (2)Ratios R1/F0 and R2/F0 can be found from equations (1) and (2):R1/F0=(L+L0)/L=(1+L0/L)  (3)R2/F0=L0/L  (4)Assuming that L0/L˜0.5, R1 and R2 can be found using equations (3) and (4), as follow:R1=1.5F0; R2=0.5F0  (5)
Equation (5) shows that the force (R1) acting on the bearing at 103 (the bearing at the pump side) is three times greater than the force (R2) that acts on the remote bearing. The conventional configuration of diaphragm pump, therefore, makes the bearing at the pump side vulnerable to damage and wear. (A comparative analysis of the forces playing a role in the diaphragm pump subject of the present invention is described in connection with FIG. 9.)